linux-ext4 - Re: [RFC PATCH v2 3/5] ext4: data=journal: write-protect pages on submit inode data buffers callback

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Message-ID: <20200819104139.GJ1902@quack2.suse.cz>
Date:   Wed, 19 Aug 2020 12:41:39 +0200
From:   Jan Kara <jack@...e.cz>
To:     Mauricio Faria de Oliveira <mfo@...onical.com>
Cc:     Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org,
        dann frazier <dann.frazier@...onical.com>,
        Mauricio Faria de Oliveira <mauricio.foliveira@...il.com>,
        Jan Kara <jack@...e.com>
Subject: Re: [RFC PATCH v2 3/5] ext4: data=journal: write-protect pages on
 submit inode data buffers callback

On Wed 19-08-20 10:44:21, Jan Kara wrote:
> I was thinking about this and we may need to do this somewhat differently.
> I've realized that there's the slight trouble that we now use page dirty
> bit for two purposes in data=journal mode - to track pages that need write
> protection during commit and also to track pages which have buffers that
> need checkpointing. And this mixing is making things complex. So I was
> thinking that we could simply leave PageDirty bit for checkpointing
> purposes and always make sure buffers are appropriately attached to a
> transaction as dirty in ext4_page_mkwrite(). This will make mmap writes in
> data=journal mode somewhat less efficient (all the pages written through
> mmap while transaction T was running will be written to the journal during
> transaction T commit while currently, we write only pages that also went
> through __ext4_journalled_writepage() while T was running which usually
> happens less frequently). But the code should be simpler and we don't care
> about mmap write performance for data=journal mode much. Furthermore I
> don't think that the tricks with PageChecked logic we play in data=journal
> mode are really needed as well which should bring further simplifications.
> I'll try to code this cleanup.

I was looking more into this but it isn't as simple as I thought because
get_user_pages() users can still modify data and call set_page_dirty() when
the page is no longer writeably mapped. And by the time set_page_dirty() is
called page buffers are not necessarily part of any transaction so we need
to do effectively what's in ext4_journalled_writepage(). To handle this
corner case I didn't find anything considerably simpler than the current
code.

So let's stay with what we have in
ext4_journalled_submit_inode_data_buffers(), we just have to also redirty
the page if we find any dirty buffers.

								Honza

-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR